Abstract Rheumatoid arthritis (RA) is a chronic inflammatory disease that primarily affects the joints. Autoreactive immune cells enter articular joints as well as skeletal muscle, and inflammation of the synovial membrane damages the cartilage causing pain and discomfort. Disability rates from RA reach 40% from 5 to 10 years after diagnosis. In addition to disease activity and pain, muscle strength has a major impact on disability in RA patients. Skeletal muscle clearly contributes to RA disability, but the mechanisms by which this occurs are not well understood. Since animal models and cell culture systems are imperfect replicates of human disease and do not recapitulate biologic and physiologic features of human skeletal muscle, we propose to use a recently developed engineered electrically responsive, contractile human muscle tissues (myobundles) to model RA in vitro and identify features of the disease phenotype. These human myobundles exhibit structural hallmarks of native skeletal muscle including aligned architecture, multinucleated and striated myofibers, and a satellite cell pool. Over four weeks in culture, they contract spontaneously and respond to single or high frequency electrical stimuli with twitch and tetanic contractions; myobundles maintain functional acetylcholine and ?-adrenergic receptors and undergo structural and functional maturation. Preliminary data with myoblasts derived from RA patients indicate that RA reduces the ability of myofibers to form and produce significant contractile force, even after removal of excess fibroblasts. To establish the in vitro system as a model to test therapies for the disease in skeletal muscle, we will extend our preliminary results and test the hypotheses hypotheses that (1) engineered skeletal muscle myobundles derived from cells of rheumatoid arthritis patients show reduced capacity for repair and differentiation and reduced force production; (2) the decrement in differentiation and function is due to select myokine and cytokine production; and (3) addition cytokines and myokines secreted by RA skeletal muscle myofibers can induce the RA phenotype in healthy myoblasts. In Aim 1, we will extend our preliminary results to show that the engineered skeletal muscle myobundles reproduce the disease phenotype and exhibit reduced maturation and capacity for force generation. Myobundles prepared with myoblasts from healthy aged-matched individuals will serve as controls. Results will be compared to immunohistochemical staining of biopsy samples to establish cellular changes in RA muscle. In Aim 2, we will use an unbiased approach and measure proteins and cytokines released from RA myobundles and apply statistical model to determine the relationship between features of RA phenotype identified in Aim 1 and the levels of myokines and cytokines. In Aim 3, we will that the hypothesis that the abnormal secretion of myokines and cytokines by RA muscle is responsible for the disease phenotype.